Effect of inorganic-to-organic mass ratio on the heterogeneous OH reaction rates of erythritol: implications for atmospheric chemical stability of 2-methyltetrols

Abstract

Abstract. The 2-methyltetrols have been widely chosen as chemical tracers for isoprene-derived secondary organic aerosols. While they are often assumed to be relatively unreactive, a laboratory study reported that pure erythritol particles (an analog of 2-methyltetrols) can be heterogeneously oxidized by gas-phase OH radicals at a significant rate. This might question the efficacy of these compounds as tracers in aerosol source-apportionment studies. Additional uncertainty could arise as organic compounds and inorganic salts often coexist in atmospheric particles. To gain more insights into the chemical stability of 2-methyltetrols in atmospheric particles, this study investigates the heterogeneous OH oxidation of pure erythritol particles and particles containing erythritol and ammonium sulfate (AS) at different dry inorganic-to-organic mass ratios (IOR) in an aerosol flow tube reactor at a high relative humidity of 85 %. The same reaction products are formed upon heterogenous OH oxidation of erythritol and erythritol–AS particles, suggesting that the reaction pathways are not strongly affected by the presence and amount of AS. On the other hand, the effective OH uptake coefficient, γeff, is found to decrease by about a factor of ∼20 from 0.45±0.025 to 0.02±0.001 when the relative abundance of AS increases and the IOR increases from 0.0 to 5.0. One likely explanation is the presence of dissolved ions slows down the reaction rates by decreasing the surface concentration of erythritol and reducing the frequency of collision between erythritol and gas-phase OH radicals at the particle surface. Hence, the heterogeneous OH reactivity of erythritol and likely 2-methyltetrols in atmospheric particles would be slower than previously thought when the salts are present. Given 2-methyltetrols often coexist with a significant amount of AS in many environments, where ambient IOR can vary from ∼1.89 to ∼250, our kinetic data would suggest that 2-methyltetrols in atmospheric particles are likely chemically stable against heterogeneous OH oxidation under humid conditions.

Effects of liquid–liquid phase separation and relative humidity on the heterogeneous OH oxidation of inorganic–organic aerosols: insights from methylglutaric acid and ammonium sulfate particles

“…This could increase the probability that 3-MGA molecules collide with OH radicals at or near the aerosol surface, leading to a higher overall rate of reactions. For instance, Xu et al (2020) have performed molecular dynamics (MD) simulations to demonstrate that upon heterogeneous reaction a gas-phase OH radical has to be first absorbed by the aerosol, and an absorbed OH radical would then need to collide with an organic molecule many times by diffusion before the reaction occurs at the aerosol surface. A higher concentration of organic molecules could reduce the distance between the OH radical and its neighboring organic molecules.…”

“…Additional uncertainties in heterogeneous reactivity of organic compounds arise when these droplets undergo phase separation. Recent studies suggest that these organic-inorganic droplets can undergo liquid-liquid phase separation (LLPS) depending on environmental conditions (e.g., RH and temperature) and aerosol composition (including different types of inorganic salts, the average oxygento-carbon (O : C) elemental ratio of organic compounds and OIR) (Bertram et al, 2011;Zuend and Seinfeld, 2013;You et al, 2014;Qiu and Molinero, 2015;Charnawskas et al, 2017;Losey et al, 2018;Freedman, 2020). These phaseseparated droplets exhibit two distinct liquid phases: typically, an inorganic-rich inner phase and an organic-rich outer phase as depicted in Fig.…”

Viscosity and phase state of aerosol particles consisting of sucrose mixed with inorganic salts

“…Different types of SOAs generated in environmental chambers and flow reactors have been examined. The dynamic viscosities of some types of SOA particles (e.g., generated from toluene and diesel vapors) ranged from 10 −3 to 10 8 Pa s, which is approximately equal to the range of viscosities between that of water and tar pitch, depending on the RH, oxidation state, and chemical composition (Mikhailov et al, 2009;Virtanen et al, 2010;Vaden et al, 2011;Koop et al, 2011;Saukko et al, 2012;Renbaum-Wolff et al, 2013a;Hao et al, 2014;Kidd et al, 2014;Bateman et al, 2015;Athanasiadis et al, 2016;Grayson et al, 2016Grayson et al, , 2017Hinks et al, 2016;Hosny et al, 2016;Rothfuss and Petters, 2017;Yli-Juuti et al, 2017;Petters et al, 2019;Gervasi et al, 2020). In addition to the viscosity of SOA, the viscosity effects of inorganic salts should also be investigated to better understand the viscosities and phase states of atmospherically relevant mixed particles.…”

“…The viscosity of aerosols can vary based on the RH and related water uptake, chemical composition, and temperature, with effects on the size distribution (Shiraiwa et al, 2013;Zaveri et al, 2014Zaveri et al, , 2018, mass concentration of aerosol particles (Shiraiwa and Seinfeld, 2012;Yli-Juuti et al, 2017;Kim et al, 2019), ice nucleation efficiency (Murray et al, 2010;Ladino et al, 2014;Knopf et al, 2018), and crystallinity of salts (Murray, 2008;Song et al, 2013;Ji et al, 2017;Wang et al, 2017). Depending on the viscosity, the phase state of aerosol particles can be determined; commonly the dynamic viscosity of a liquid is characterized as being less than 10 2 Pa s, that of a semi-solid is between 10 2 and 10 12 Pa s, and that of a glassy or crystalline solid is typically greater than 10 12 Pa s (Zobrist et al, 2008;Koop et al, 2011;Reid et al, 2018).…”

“…Most studies on this topic have focused on the determination of the viscosities and phase state of secondary organic aerosols (SOAs) (Mikhailov et al, 2009;Virtanen et al, 2010;Koop et al, 2011;Vaden et al, 2011;Saukko et al, 2012;Renbaum-Wolff et al, 2013;Hao et al, 2014;Kidd et al, 2014;Bateman et al, 2015;Athanasiadis et al, 2016;Grayson et al, 2016;Hinks et al, 2016;Hosny et al, 2016;Yli-Juuti et al, 2017;Petters et al, 2019;Wallace and Preston, 2019;Gervasi et al, 2020;Schmedding et al, 2020). Different types of SOAs generated in environmental chambers and flow reactors have been examined.…”